Hello,
I have a problem viewing my file after uploading. I designed it on AutoCAD 3D to have hollow tubes. After uploading to Shapeways, became enclosed on each end and I can't see it hollow. I already downloaded netfabb basic and other STL viewer, there I can see it hollow. The only problem is after uploading, I can't see it being hollow.

Hi. Thanks for the help. I am actually not so expert and I have just started on designing 3D. What do you mean by fixing open edges? Does that mean that the surfaces of these two are not joined together?
And did the upload process fixed it by filling in the gap in between these two surfaces?

Perfect. Thanks for that big Clue. The combination of the loft and thicken is the root cause. The thicken did not follow the base plane and rather did something like an offset from the surface to create the thickness. The direction of the Thicken created the angle thus creating an Open edge.